Antenna system



M y 8, 1 5 Y L. R. DOBLER ETAL 2,551,586

ANTEIiNA SYSTEM I Filed Aug. 9, 1945 LEE R. DOBLER LLOYD J. MONEY.

QWIQ Patented May 8, 1951 UNITED STATES ANTENNA SYSTEM Lee R. Dobler and Lloyd J. Money, United States Navy Application August 9, 1945, Serial No. 609,906

Claims.

This invention relates in general to multi-purpose antenna systems and in particular to parasitic elements therefor.

In radio and the allied arts it frequently is desirable to provide a m'ulti-purpose antenna system in which a number of antenna arrays having distinct functions are contained on a, common frame or support. In such cases a first array may act as an obstruction to a second array whereby reactive loading of one or more of the radiating elements of the second array may result. In this case optimum impedance match between the transmission line and the radiating elements of the second array, especially when an extended frequency range is to be covered by the second array, is unobtainable.

It is an object of this invention to provide, in an antenna system in which at least one of the radiating elements is reactively loaded by an obstruction, a means for obviating the effects of such reactive loading.

Another object of this invention is to provide a means for obviating reactive loading of a radiating element by an interfering element or surface placed in proximity thereto and thereby maintaining the free space radiation pattern of th radiating element.

Another object of this invention is to provide an antenna system which carries a number of functionally distinct antenna arrays on a common antenna frame.

Other objects and features of the present invention will become apparent upon a careful consideration of the accompanying drawings and the detailed discussion which follows:

Fig. 1 shows an end-view of an antenna system embodying the principles of the principal invention, and

Fig. 2 shows a front view taken from the direction of maximum radiation by the antenna system of Fig. 1.

A dual purpose antenna system constructed according to the teachings of this invention is shown in Figs. 1 and 2. In particular the antenna system of the drawing comprises two functionally distinct arrays. The first is a radar antenna designed to produce a pair of divergent but slightly overlapping beam patterns and comprises, in part, two cylindrical parabolic reflecting surfaces held in juxtaposed relationship by a pair of end plates 20 and 2| welded to each end of the parabolas l0 and II. Each of the parabolas is made, for example, from perforated sheet metal and they are so oriented with respect to one another that their axes are slightly divergent to thereby provide divergent beam patterns. At the focal line of each of the parabolic sections I0, I I are a number or horizontal half-wavelength radiating elements [3 located approximately a wavelength apart, center to center. The manner in which the radiating elements I3 are supported at the focal lines of the respective parabolas and their mode of excitation being in accordance with the teaching of the prior art. With this arrangement it was found that excessive energy was present in side lobes of the radiation pattern. It was further found that these side lobes could be effectively suppressed by the incorporation of a, sub-antenna assembly comprising a third smaller cylindrical parabolic reflecting surface I 2 with a pair of half wave radiating elements I 3A similarly supported and excited at its focal line. Each end of the parabola I2 is secured to a yoke member I6, shown partly cut-away in Fig. 1. The parabola I2 is then positioned in front of the center of the main reflectors l0, H by the attachment of the yokes IE to a retaining member l8 fixed at the intersection of parabolas l0, ll. Also supporting parabola I2 and providing a path for energy to the radiating elements l3A is a coaxial tubing, the rigid outer conductor of which is shown at IT. The coaxial line I! is mounted at the center point of the antenna assembly and extends through the screens in, H at their line of intersection to the main transmission line, not shown. The exact position and size of the reflector I2 and its elements l3A, as well as the amount and phasing of the energy applied to elements l3-A are criti cal for optimum side lobe suppression but are not within the scope of this invention.

The second array comprises a pair of halfwavelength vertical end radiators l4 and a centrally disposed radiator I4-A. This array is designed to provide a vertically polarized radiation pattern at a lower frequency than the first array and is arranged to function, for example, in connection with an identification system, of any suitable type known to the art and whose function is to determine whether an object seen by the radar is friendly or unfriendly. For dependable operation of the identification system it is of utmost importance to positively establish the fact that the object being challenged by the system is the same as that at which the radar antenna is directed. This means that at least three halfwavelength spaced. dipoles I4 and I l-A are required in order to obtain the desired directivity in the identification system antenna beam pattern. To facilitate feeding the elements and prevent objectionable back radiation it is found desirable to mount the radiating elements l4, I4A upon the line of intersection 18 of the parabolas ii), i I, and supply energy to them by a suitable transmission means, not shown, fed through openings in the reflector screen. In the preferred case the one half-wavelength dipoles l4 and M-A are current fed so as to produce avolta-ge minimum at these center points permitting a rigid; conductive connection by welding or clamping the dipole center points to the intersection of the, parabolas l and I I.

It is apparent that in the last antenna assembly it is not possible to provide a symmetrical array comprising at least three radiating elements in the vertical set I4, lQ-A without locating one of the elements in proximity to the lobe suppressing reflector [2. The presence of this obstruction does not adversely affect the radiation field patterns of the combination of the elements M, but it does produce reactive loading on the partially covered element I4-A. By proper impedance matching of the radiating element to the line feeding it, satisfactory operation with a standing wave ratio of E minimum to E maximum in the transmission line greater than 0.5 is possible at a particular frequency.

Normally the identification equipment employing the radiators I4, l l.A must function over an extended frequency range, therefore these elements are constructed of large diameter tubing to give the broadband characteristics. The reactive loading placed upon element I4A by the reflector [2 causes that element to be sharply resonant so that operation over an extended fre-. quency range is not possible.

It was found that this reactive loading could be reduced substantially by interposing a parasitic element I tuned to approximately the same wavelength as radiator l4A between the reflector l2 and the element l4--A. The parasitic element I5 is mounted, clamped, for example to the outer conductor ll of the coaxial tubing feeding elements l3A. Element I5 is not excited directly but is a true parasite inasmuch as it receives its excitation from element I4A. The insertion of the parasite 15 produces two resonant points in the frequency response characteristics of the central element MA with little mis-tuning occurring between. The spacing of these resonant points is a function of the diameter of the radiator ldA and that of the parasitic element l5, being less where smaller diameter elements are employed. The spacing of the parasite 15 from radiator I l-A and from the back surface of reflector I2 are both of importance but are best determined empirically. In the typical case here chosen, the spacing between the parasite I5 and the radiator !4A is approximately 8% of the average wavelength. Also in this typical case the length of the end radiators IA is approximately 48% of the average wavelength. The length of the center radiator l l-A is approximately 45% of the average wavelength, while the parasite I5 is slightly shorter (or tuned to a higher frequency) than the center radiator I4- A. Under these conditions it is possible to obtain satisfactory operation without retuning over a frequency range as great as 10% of the average frequency. 7

The exact action of the parasitic element IS in reducing reactive loading of radiator I4A is not known, however, it is believed that the radiator MA and the parasite l5 function as over-coupled tuned circuits producing the familiar dual-peaked frequency response.

The addition of the parasitic element :5 and the vertical radiators My, l lA does not materially affect the operation of the first antenna array since the respective planes of polarization bear degree relationships to each other.

From the foregoing discussion it is apparent that the principles of this invention may be applied to other types of antenna systems where reactive loading of one or more elements is produced as, a result of the proximity of an interfering surface. It would also be applicable where flat rather than parabolic reflectin surfaces are employed for the antennas l3, l3-A.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. In an antenna system having first and second arrays positioned on a common antenna frame such that the first array forms an obstrucs tion to at least one radiating element of the sec.- ond array whereby reactive loading of said radiating element results, a parasitic element positioned intermediate said radiating element and said first array, said parasitic element being tuned to substantially the same frequency as said .ra, diating element.

2. In an antenna system having first and second arrays positioned on a common antenna frame such that th first array forms an obstruction to at least one radiating element of the second array whereby reactive loading of said radiating element results, a parasitic element positioned intermediate said radiating element and said first array at a fraction of a wavelength from said radiating element, said parasitic element being tuned to substantially the same frequency as said radiating element.

3. In an antenna system having first and second arrays positioned on a common antenna frame such that the first array forms an 0bstruction to at least one radiating element of the second array whereby reactive loading of said radiating element results, a parasitic element positioned intermediate said radiating element and said first array at a fraction of a wavelength from said radiating element, said parasitic element being tuned to a frequency slightly higher than said radiating element.

4. An antenna system comprising, a first antenna assembly having a reflector and associated array of radiating elements, a second. antenna assembly having a reflector and associated array of radiating elements, said second assembly being positioned to the radiation side of said first assembly, a third and functionally distinct array of radiating elements, said third array positioned intermediate said first and second antenna as-. semblies and a parasitic element positioned intermediate said third array and said second as.- sembly, said parasitic element being tuned to substantially the same frequency as the radiating elements of said third array. 7

5. An antenna system comprising, a first antenna assembly having a reflector and associated array of radiating elements, a second antenna assembly having a reflector and associated array of radiating elements, said second assembly being positioned to the radiation side of said first assembly, a third and functionally distinct array of radiating elements, said third array positioned intermediate said first and second antenna assemblies, and a parasitic element positioned intermediate said third array and said second assembly at a fraction of a wavelength from said third array, said parasitic element being tuned to substantially the same frequency as the radiating elements of said third array.

LEE R. DOBLER. LLOYD J. MONEY.

6 REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,938,066 Darbord Dec. 5, 1933 2,054,896 Dallenbach Sept. 22, 1936 2,292,791 Mims Aug. 11, 1942 2,380,519 Green Jul 31, 1945 2,458,885 Warren Jan. 11, 1949 2,471,284 Rea May 24, 1949 

